Mufflers have been developed for the attenuation of the sound component in an exhaust gases from an internal combustion engine which employ sound-attenuating partition configurations that produce low-pressure regions or volumes within the muffler. The low-pressure volume can be the result of cancellation of identical sound frequencies by directing streams of gas to collide against each other, or can be the result of fluid flow patterns through the muffler, or both.
The flow of exhaust gases through a muffler is not in a steady stream of the type which exits a garden hose. Instead, each time an exhaust valve opens, a pulse of exhaust gases is discharged into the exhaust system. Thus, flow of exhaust gases through a muffler is comprised of a series of volume pulses in which there are fully combusted gases, live fire or burning fuel and, in some cases, fuel which is unignited and will not contact or mix sufficiently with the burning fuel to ignite.
When these exhaust components reach the muffler, the muffler partitions typically quench or retard further burning of unignited fuel rather quickly. The result is that a small volume of unignited fuel may be present in the muffler. If the muffler includes low-pressure regions or volumes, there will be a tendency for this unignited fuel to accumulate in such regions.
The presence of a low-pressure volume in a muffler, nevertheless, is highly desirable since in some muffler configurations it has been found to increase engine horsepower. It is believed that the low-pressure region in the muffler is "seen" upstream in the exhaust system to the engine. The low pressure in the muffler scavenges or accelerates the movement of exhaust gas pulses in the exhaust system. Thus, pulses proximate the low-pressure volume are accelerated toward it, which, in turn, accelerates pulses farther upstream. Finally, when the engine exhaust valve opens to exhaust gases from the cylinder, these gases are exhausted into a lower pressure exhaust system than would be present if the muffler did not have low-pressure volumes in it. This slightly lower pressure at the exhaust valve enables the same volume of gases to be exhausted from the cylinder in a slightly shorter period of time. This, in turn, allows the engine to be tuned to keep the exhaust valve closed slightly longer, which allows the engine to develop additional horsepower.
A muffler which employs these principles and has been highly effective in sound attenuation while increasing engine horsepower is the muffler of my U.S. Pat. No. 4,574,914. This muffler is now in wide-spread use by independent professional race car drivers and even by some of the manufacturer-sponsored racing teams.
While not as effective in their sound-attenuation/horsepower-increase as the muffler of U.S. Pat. No. 4,574,914, other mufflers which include partition structures that will tend to produce, in varying degrees, low-pressure volumes or regions within the mufflers include: U.S. Pat. Nos. 624,062; 1,184,431; 1,081,348; 2,071,351; 2,325,905; 2,239,549; 2,485,555; 2,667,940; 2,971,599 and British Patent No. 285,604.
The accumulation of unignited fuels in mufflers having low-pressure volumes can present a problem which ranges from annoying to potentially dangerous. Under most operating conditions such fuels are either not accumulated or are dissipated. However, under some conditions explosive detonations or rapid combustion can occur. Thus, when engines are tuned to run on a rich fuel mix, or when turbo-chargers are employed and the engine is decelerating, unignited fuel is more likely to accumulate and be ignited or explode in low-pressure regions of mufflers. Either live fire or accumulated muffler heat appears to ignite the accumulated fuel. Such muffler explosions can range from disconcerting popping sounds during deceleration to violent explosions which damage the muffler and exhaust system.
In turbo-charged production cars using the muffler of my U.S. Pat. No. 4,574,914, for example, muffler popping has been observed during engine deceleration.
In high-performance racing cars which burn alcohol, as well as gasoline, mufflers constructed in accordance with U.S. Pat. No. 4,574,914 have occasionally experienced sufficiently violent explosions or fuel detonations to burst the muffler casing. While occurring only under the just the right combination of conditions, such muffler explosions are, of course, highly undesirable.